This invention pertains to photovoltaic devices and more particularly to multi-junction solar cells fabricated of amorphous silicon and copper indium diselenide and their alloys.
Over the years numerous solar cells have been developed which have met with varying degrees of success. Single junction solar cells are useful but often cannot achieve the power and conversion efficiency of multi-junction solar cells. Unfortunately, multi-junction solar cells and single junction solar cells have been constructed of various materials which are able to capture and convert only part of the solar spectrum into electricity. Multi-junction solar cells have been produced with amorphous silicon and its alloys, such as hydrogenated amorphous silicon carbon and hydrogenated amorphous silicon germanium, with wide and low bandgap intrinsic i-layers. Amorphous silicon solar cells have a relatively high open circuit voltage and low current but can only respond to capture and convert into electricity wavelengths of sunlight from 400 to 900 nanometers (nm) of the solar spectrum.
Copper indium disclenide (CIS) polycrystalline solar cells have a relatively low bandgab of approximately 1 eV and are able to respond, capture and convert into electricity a great spectrum of sunlight from 400 to 1350 nm. Copper indium diselenide solar cells can generate more current but at lower voltage than amorphous silicon solar cells and their alloys. Copper indium diselenide polycrystalline solar cells, however, generally are more temperature dependent than amorphous silicon solar cells and can lose as much as 60% of their power at higher temperatures in a manner somewhat similar to polycrystalline silicon solar cells.
The segments, layers or cells of multi-junction solar cells are electrically interconnected, such as by laser scribing. High current CIS polycrystalline solar cells generate greater power losses (I2R) due to the resistance at the front and rear contacts, e.g. tin oxide contacts, than do amorphous silicon solar cells. Such power losses can be partially overcome by laser scribing more scribe lines and dividing the solar cell into smaller segments, such as 60 segments of 1 cm width. More scribe lines, however, decreases the active area of utilization of the solar cell which is available to capture and convert solar energy into electricity. Furthermore, deviations, voids and imperfections in the composition of polycrystalline can adversely effect the performance of polycrystalline solar cells.
Current output of a photovoltaic device is maximized by increasing the total number of photons of differing energy and wavelength which are absorbed by the semiconductor material. The solar spectrum roughly spans the region of wavelength from about 300 nanometers to about 2200 nanometers, which corresponds to from about 4.2 eV to about 0.59 eV, respectively. The portion of the solar spectrum which is absorbed by the photovoltaic device is determined by the size of the bandgap energy of the semiconductor material. Crystalline silicon (c-Si) has a bandgap energy of about 1.1 eV. Solar radiation (sunlight) having an energy less than the bandgap energy is not absorbed by the semiconductor material and, therefore, does not contribute to the generation of electricity, current, voltage and power, of the photovoltaic device.
Solar cells and other photovoltaic devices convert solar radiation and other light into usable electrical energy. The energy conversion occurs as the result of the photovoltaic effect. Solar radiation (sunlight) impinging on a photovoltaic device and absorbed by an active region of semi-conductor material, e.g. an intrinsic i-layer of amorphous silicon, generates electron-hole pairs in the active region. The electrons and holes are separated by an electric field of a junction in the photovoltaic device. The separation of the electrons and holes by the junction results in the generation of an electric current and voltage. The electrons flow toward the region of the semiconductor material having an n-type conductivity. The holes flow toward the region of the semiconductor material having a p-type conductivity. Current will flow through an external circuit connecting the n-type region to the p-type region as long as light continues to generate electron-hole pairs in the photovoltaic device.
Single-junction devices comprise three layers. These are p- and n-layers which are extrinsic or doped and i-layer which is intrinsic or undoped (at least containing no intentional doping). The i-layer is much thicker than the doped layers. This is because mainly light absorbed in the i-layer is converted to electrical power which can be used in an external circuit. The thickness of the i-layer (sometimes called the absorber layer) determines how much light is absorbed. When a photon of light is absorbed in the i-layer it gives rise to a unit of electrical current (an electron-hole pair). However, this electrical current will go nowhere on its own. Hence, the p- and n-layers. These layers, which contain charged dopant ions, set up a strong electric field across the i-layer. It is this electric field which draws the electric charge out of the i-layer and sends it through an external circuit where it can do work (i.e. power a light bulb).
An amorphous silicon solar cell is comprised of a body of hydrogenated amorphous silicon (a-Si:H) material, which can be formed in a glow discharge of silane. Such cells can be of the type described in U.S. Pat. No. 4,064,521 entitled Semiconductor Device Having A Body Of Amorphous Silicon which, issued to David E. Carlson on Dec. 20, 1977. Within the body of the cell there is an electric field which results from the different conductivity types of the semiconductor regions comprising the body.
Amorphous silicon solar cells are often fabricated by the glow discharge of silane (SiH4). The process of glow discharge involves the discharge of energy through a gas at relatively low pressure and high temperature in a partially evacuated chamber. A typical process for fabricating an amorphous silicon solar cell comprises placing a substrate on a heated element within a vacuum chamber. A screen electrode, or grid, is connected to one terminal of a power supply, and a second electrode is connected to the substrate. While silane, at low pressure, is admitted into the vacuum chamber, a glow discharge is established between the two electrodes and an amorphous silicon film deposits upon the substrate.
Amorphous hydrogenated silicon (a Si:H) based solar cell technology is currently the leading candidate for large area, low-cost photovoltaic applications. The basic device structure is a single p-i-n junction or an n-i-p junction in which all layers are traditionally amorphous and are made in a continuous plasma deposition process.
The substrate of the solar cell can be made of glass or a metal, such as aluminum, niobium, titanium, chromium, iron, bismuth, antimony or steel. If a glass substrate is used, a transparent, conductive coating, such as tin oxide (SnO2) can be applied to the lass substrate prior to forming the amorphous silicon. A metallic contact can be formed on the back of the substrate.
The doped layers in the device play a key role in building up the strong internal electric field across the i-layer, which is the predominant force in collecting photocarriers generated in the i-layer. In particular, the doped layers in the recombination junction of a multi-junction solar cell have to support large electric fields extending into the intrinsic layers, in addition to the high field in the recombination junction itself. The interface region must promote efficient recombination of electrons, generated in the first i-layer, with holes from the second i-layer. Also, the tunnel junction layers should provide minimal optical absorption. However, the electrical properties of amorphous doped layers are relatively poor as compared to their crystalline counterparts. For instance, the conductivities are typically only xcx9c1xc3x9710xe2x88x926(xcexa9xc2x7cm)xe2x88x921 for a-Si:H p-layer and xcx9c1xc3x9710xe2x88x924 (xcexa9xc2x7cm)xe2x88x921 for the n-layer. This is due partly to the low carrier mobilities in a-Si:H and partly to the low doping efficiencies in the disordered material. Moreover, the extremely high densities of tail states in amorphous materials prevent the Fermi levels from being too close to the band edges. The typical conductivity activation energies for a-Si:H p-layers and n-layers are xcx9c0.4 eV and xcx9c0.2 eV, respectively, thereby limiting the open circuit voltage of the a-Si:H solar cells to xcx9c0.9 V given its bandgap of xcx9c1.75 eV.
At open circuit conditions, the voltage of the multi-junction solar cell should ideally be the sum of voltage developed across each p-i-n junction if there is no voltage dropped across the tunnel junctions. However, for non-ideal tunnel junctions a significant voltage in opposite polarity with that generated by the p-i-n junctions in the device can occur due to accumulation of photocarriers near the tunnel junction, and thus reduce the open circuit voltage.
Another important quality for the doped layers used in solar cells, besides good electrical properties, is low optical absorption. In contrast to single crystalline devices where p-n junctions can be used, photons absorbed in amorphous doped layers can be lost because the diffusion length of photo-carriers is extremely short in those layers. This requirement is especially important for the p-layer through which light enters into the device. It is partly for this reason that amorphous silicon carbon (a-SiC:H) p-layers with an optical bandgap xcx9c2.0 eV have often been used instead of amorphous silicon (a-Si:H) p-layers.
Converting amorphous a-Si:H to xcexcc-Si also lowers the optical absorption in the short wavelength region due to the selection rule for optical transition in crystalline grains. The absorption coefficient of xcexcc-Si p-layer is higher than that of the amorphous silicon carbon (a-SiC:H) p-layer typically used in the solar cells. Doped microcrystalline silicon (xcexccsi) represents a very attractive alternative for a-Si:H based solar cells not only because of its much improved electrical and optical properties but also its compatibility with the enhanced plasma chemical vapor deposition process. However, except for very few reported successes, microcrystalline silicon so far has not been widely used in amorphous silicon (a-Si:H) solar cells, at least for commercial applications. The main difficulties are perhaps in making extremely thin layers of xcexcc-Si(xe2x89xa6100 xc3x85), which is necessary in order to reduce the optical loss, and in alloying with carbon for raising the optical bandgap.
The bulk properties of microcrystalline silicon (xcexcc-Si) are very different from those of extremely thin layers made under the same microcrystalline condition. Therefore, the bulk properties of microcrystalline silicon have little relevance to the application of xcexcc-Si in solar cells where only ultra-thin layers are used. When examining the thickness dependencies of conductivity and activation energy for films made under conventional microcrystalline p-layer conditions, it can be observed that the film properties change dramatically when the microcrystalline thickness is below xcx9c1000 xc3x85. This is not surprising because it is well known that nucleation is critical in forming crystalline grains on a heterogeneous substrate. Also, the properties of those films made under xe2x80x9cmicrocrystallinexe2x80x9d conditions may be strongly substrate dependent, especially for ultra-thin layers. Furthermore, whether the substrate is conducting or insulating apparently also influences the initial nucleation of xcexcc-Si, at least in D.C. plasma. In the past, it has been found that microcrystalline silicon (xcexcc-Si) forms much more readily on stainless steel substrates than on an amorphous silicon (a-Si:H) layer.
Solar cells made of copper indium diselenide (CIS) and its alloys, such as copper indium gallium selenide (CIGS) can be useful, but usually by themselves produce low voltages and high power (I2R) losses.
It is, therefore desirable to provide better processes for producing improved monolithic multi-junction solar cells with amorphous silicon and CIS and their alloys.
Monolithic solar cells are produced which are able to capture a broader spectrum of sun light and convert and harness a greater amount of solar energy into electricity. Advantageously, the monolithic solar cells and process for their production are efficient, effective, reliable, and economical. The monolithic solar cells can comprise: single junction solar cells, tandem solar cells, triple junction solar cells, or other multi-junction solar cells.
As used in this application, the term xe2x80x9cmonolithicxe2x80x9d means a solar cell comprising a front contact and a rear-contact. The segments, layers, or cells of a monolithic solar cell are electrically and optically connected to each other to form one solar cell.
The process for producing the inventive monolithic solar cells comprises coupling an amorphous silicon thin film semiconductor to a copper indium polycrystalline semiconductor by: depositing the copper indium polycrystalline semiconductor on a vitreous substrate or a metal substrate, depositing an n-type conductor on the copper indium polycrystalline semiconductor, and depositing the amorphous silicon thin film semiconductor on the n-type conductor. Preferably, the copper indium polycrystalline semiconductor is deposited by evaporation. The copper indium polycrystalline semiconductor can contain 0% to 24% by weight gallium and preferably comprises copper indium diselenide (CIS) or copper indium gallium selenide (CIGS). In the preferred process, a rear contact is deposited on the substrate before the copper indium polycrystalline semiconductor is deposited, and a front contact is positioned and deposited on the amorphous silicon thin film semiconductor.
In one embodiment, the n-type conductor comprises cadmium sulfide. The cadmium sulfide can be deposited by solution growth, sputtering or evaporation. A transparent conductive oxide, such as zinc oxide or tin oxide, can be deposited on the cadmium sulfide by low pressure chemical vapor deposition (LP CVD). In another embodiment, the n-type conductor comprises microcrystalline n-doped amorphous silicon. The microcrystalline n-doped amorphous silicon can be deposited by enhanced plasma chemical vapor deposition (EP CVD), also referred to as plasma enhanced chemical vapor deposition (PE CVD).
The amorphous silicon thin film semiconductor can comprise a p-i-n or n-i-p semiconductor and can be formed of hydrogenated amorphous silicon, hydrogenated amorphous silicon carbon, and/or hydrogenated amorphous silicon germanium. The amorphous silicon thin film semiconductor can be deposited by enhanced plasma chemical vapor deposition.
In a preferred process for producing triple junction solar cells, an amorphous silicon germanium semiconductor is deposited on the n-type conductor by EP CVD, and an amorphous silicon conductor is deposited by EP CVD on the amorphous silicon germanium semiconductor.
The monolithic solar cell can have a metal substrate, made of steel, iron, aluminum, niobium, titanium, chromium, bismuth, or antimony, but preferably comprises a transparent vitreous substrate made of glass. The rear contact can be made of molybdenum, aluminum, silver, zinc oxide or tin oxide. The front contact can be made of zinc oxide, tin oxide or other transparent conductive oxides.
The inventive solar cells and process produced unexpected surprisingly good results. The invention combines a copper indium polycrystaline semiconductor with an amorphous silicon thin film semiconductor in a monolithic manner such that the resultant photovoltaic devices have a high open circuit voltage and a high short circuit current density. The amorphous silicon semiconductor responds and effectively converts up to and primarily from 400 to 900 nanometers (nm) of light to electricity. The CIS or CIGS polycrystalline semiconductor responds and effectively converts the remaining 900-1400 nrm of light to electricity. The CIS or CIGS polycrystalline semiconductor combines and cooperates with the amorphous silicon semiconductor to provide synergistic results. The inventive process and solar cells provide better conversion efficiency, fill factor (FF), short circuit current density, and open circuit voltage (Voc). Advantageously, the novel process and solar cells are economical, attractive and effective.
The multijunction solar cells can be fabricated by forming a microcrystalline sandwich with a n-type semiconductor (conductor) comprising a microcrystalline tunnel junction layer between the CIS or CIGS polycrystalline solar cell and an adjoining n-i-p amorphous silicon-containing solar cell. The amorphous silicon-containing solar cell has a positively doped p-layer, an active instrinic i-layer, and a negatively doped n-layer. A tunnel junction or recombination junction connects the polycrystalline back solar cell to he amorphous silicon-containing solar cell. The tunnel junction can comprise a negatively doped layer from one of the solar cells, a positively doped layer from the other solar cell, and at least one intermediate tunnel junction layer positioned between the polycrystaline and amorphous silicon-containing second solar cells.
The tunnel junction layer can be fabricated by etching and treating one of the doped layers in the tunnel junction to form an etched surface thereon and, thereafter, nucleating from the etched surface to form and grow a microcrystalline tunnel junction layer. As used in this application, the term xe2x80x9cnucleatingxe2x80x9d means the initial growth phase of microcrystalline layers. Etching can comprise enhanced plasma chemical vapor deposition with an enchant (treatment material), such as hydrogen, deuterium, HD, helium, and argon. Preferably, etching comprises hydrogen etching alone by DC or RF enhanced plasma chemical vapor deposition while preventing substantial optical and electrical damage to the doped layers. Desirably, for best results, silane or other feedstock is prevented from being deposited with the enchant.
Nucleation from the etched surface can be provided to accelerate microcrystalline growth. Microcrystalline nucleation can be accomplished by enhanced plasma chemical vapor deposition with a dopant and a feedstock diluted with a diluent. The dopant can be: a negative dopant comprising a n-type dopant, such as phosphine (PH3) or other phosphorous-containing compounds; or a positive dopant comprising a p-type dopant, such as diborane (B2H6) BF3, or other boron-containing compounds. The feedstock can be: silane (SiH4), disilane(Si2H6), tetramethyl silane, Si(CH3)4, SiF4, SiHF3, SiH2Cl4, CHN(SiH3)4-N wherein N is an integer in the range of 0 to 3, a carbon based feedstock; or a germanium based feedstock. The feedstock can also have the general formula SiNH2N+2xe2x88x92MYM wherein:
Si=silicon
H=hydrogen or deuterium
Y=a halogen [fluorine (F), chlorine (Cl), bromine (Br), Iodine (I), etc.]
N=positive integerxe2x89xa71
M=positive integer; and
2N+2xe2x88x92Mxe2x89xa70.
The diluent can be hydrogen (H2), deuterium (D2), or HD. The dilution ratio of the diluent to the feedstock can range from about 50:1 to about 200:1.
Plasma enhanced chemical vapor deposition (PECVD) can be by: cathodic direct current (DC) glow discharge, anodic DC glow discharge, radio frequency (RF) glow discharge, very high frequency (VHF) glow discharge, alternating current (AC) glow discharge, or microwave glow discharge. Plasma enhanced chemical vapor deposition of microcrystalline layers can be accomplished at a temperature ranging from 80-300xc2x0 C., at a pressure ranging from 0.5-5 Torr, with a dilution ratio of diluent to the feedstock (deposition gas) ranging 50:1 to 200:1.
As discussed above, the tunnel junction of the multi-junction solar cell can have an etched surface and at least one microcrystalline tunnel junction layer sandwiched between the doped layer of one solar cell and an opposite doped layer of the other solar cell. The etched surface can be a hydrogen plasma etched surface, such as a n-doped amorphous silicon surface or a p-doped amorphous silicon surface. The microcrystalline tunnel junction layer can be a p-type microcrystalline layer and/or an n-type microcrystalline layer and the microcrystalline layer can be microcrystalline silicon carbon, microcrystalline silicon germanium, or microcrystalline silicon. Desirably, the microcrystalline layer can have a thickness of 50-120 xc3x85, preferably from 80-100 xc3x85 for best results. The tunnel junction can comprise an n-type doped non-crystalline amorphous layer, an n-type microcrystalline tunnel junction layer, and a p-type doped non-crystalline amorphous layer. Tunnel junction can also comprise a p-type microcrystalline tunnel junction layer in lieu of or in addition to the n-type microcrystalline tunnel junction layer.
Preferably, the solar cells include an n-i-p amorphous silicon-containing solar cell and a copper indium diselinide (CIS) solar cell or a copper indium gallium selenide (CIGS) solar cell. At least one of the layers of the amorphous silicon-containing solar cell, i.e. the n layer, i layer, and/or p layer, in one the solar cells comprises: hydrogenated amorphous silicon, hydrogenated amorphous silicon carbon, or hydrogenated amorphous silicon germanium.
A more detailed explanation of the invention is provided in the following description and appended claims taken in conjunction with the accompanying drawings.